[Rock-dev] Making the ruby part of gui/vizkit "optional"

[Sylvain Joyeux]

I'd like to put all the dependencies in gui/vizkit that are only needed 
for the Ruby part optional.

Rationale:

  - the C++ part of vizkit is framework-independent
  - the Ruby part is (mostly) framework independent
  - Rock libraries should be compilable without the framework

Thoughts ?

Sylvain